feat(M1): TCP/TLS control plane -- auth, channels, ephemeral text

Implements the full M1 milestone. Two clients authenticate over TLS 1.3
(guest + Argon2id password) and exchange channel + private text messages
through a real server. All five ctest --preset m1-dev tests pass in ~1 s.

Key components added:
- vcpkg baseline + m1-dev preset (protobuf/mbedTLS/libsodium/asio/sqlite3)
- FrameCodec feed+emit, encode/decode_envelope, protobuf codegen
- TcpServerConn with blocking TLS handshake thread + tls_read_loop
- TlsContext (mbedTLS 1.3, ECDSA-P256 self-signed cert, TOFU on client)
- WorkerPool (3 threads, used for Argon2id)
- Database: SQLite + libsodium Argon2id, account lifecycle, bootstrap admin
- ServerIdentityManager: Ed25519 key + cert generate/persist/fingerprint
- ConnSession state machine: WaitingHello -> WaitingAuth -> Authenticated
- SessionRegistry: channel tree, user map, text routing, broadcast
- vc_client full M1 C ABI: connect/TLS/handshake/auth/text/disconnect
- voicecat-admin CLI: account add/reset/del/list
- test_m1_integration: M1 exit criterion, verified green

Bug fixed: double-framing in ConnSession::send_envelope -- encode_envelope
was adding the [4-byte len] prefix, then TcpServerConn::send_frame added
a second one, causing the client to parse [len][proto] as protobuf (silent
failure). Fixed by serializing raw protobuf bytes in send_envelope and
letting send_frame apply the single length prefix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -4,9 +4,9 @@ Auto-loaded each session. This is the **map**: build commands, architecture at a
 where everything is. For the *working method* read [`AGENTS.md`](AGENTS.md); for *what's done
 and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](docs/).
 
-> **One-line status:** M0 skeleton is complete and verified (builds + links + smoke test
-> passes). Next up is **M1** (TCP/TLS control plane, auth, channels, ephemeral text). See
-> [`PROGRESS.md`](PROGRESS.md).
+> **One-line status:** M1 control plane is complete and verified (`ctest --preset m1-dev`
+> green — 5/5 tests including full TLS auth + text relay integration test). Next up is
+> **M2** (UDP media, Opus, jitter buffer). See [`PROGRESS.md`](PROGRESS.md).
 
 VoiceCat = self-hosted native voice & text chat (TeamSpeak/Mumble-style). Plain TCP (control)
 + UDP (media), no WebRTC, encrypted by default. A shared C++ core (`libvoicecat`) drives